Solids impacted with hexavalent chromium/Cr(VI) can be difficult to treat, particularly in the case of COPR. Applicable treatment standards for Cr(VI) and/or total chromium in soils/solids can be broken into two categories, as follows:
Toxicity Characteristics:
this category refers to a determination of leachable chromium. Solids impacted with chromium are regulated by the United States Environmental Protection Agency (“USEPA”), as well as localities to prevent the improper land disposal of material that is characteristically hazardous due to leaching. The USEPA has established 5 milligrams per liter (mg/L) by the Toxicity Characteristic Leaching Procedure (“TCLP”) as the limit for total chromium (sum of Cr(VI) and Cr(III) in leachate) at/above which the material tested is considered characteristically hazardous. For such materials, Land Disposal Restrictions require a minimum 90-percent reduction in leachability through treatment, with residual leachability no more than 10-times the universal treatment standard for total chromium, which is 0.6 mg/L (as determined by TCLP). As an additional consideration for treated materials to be protective of groundwater, the USEPA has established a maximum contaminant level of 100 micrograms per liter (μg/L) for total chromium in drinking water. Separate standards specific to Cr(VI) are also being considered.
Solid Concentrations:
this category refers to a determination of chromium (Cr(III) or Cr(VI)) concentration in the impacted solids. An example of treatment standards for solid phase concentrations is provided by the state regulations in New Jersey; this state has numerous sites where chromite ore was processed and resultant COPR impacts in soil:
120,000 mg/kg Cr(III) for residential properties.
240 mg/kg Cr(VI) for residential properties, potentially being revised to 1 mg/kg.
20 mg/kg Cr(VI) for industrial properties.
The mineral chromite is the only commercial source of chromium; it is a spinel (FeO.Cr2O3). In its natural form, chromite is a mixture described by the formula (Fe2+,Mg)O.(Cr,Al,Fe3+)2O3 and it rarely contains more than 50% Cr2O3. Other minerals such as SiO2 can be present (See Habashi, F., 1997. Handbook of Extractive Metallurgy, Volume IV. pp. 1761-1812).
The acronym “COPR” is used to describe chromite or processing residues, the residues created by historical extraction of chromium from chromium (chromite) ore for industrial purposes. This was historically completed by heating pulverized ore with soda ash (Na2CO3) and lime (CaO) at around 1100° C. This would oxidize the insoluble Cr(III) to Cr(VI), making it available for extraction primarily as sodium chromate (NaCrO4). The resulting solids were disposed in a variety of manners, including use as fill material in and around the original manufacturing sites.
These solids can exhibit the following primary characteristics:
High Concentrations of Cr(VI).
After the oxidative process, the roast is a mixture of soluble salts and insoluble components including sodium chromate, sodium aluminate, magnesium oxide, sodium vanadate, iron(III) oxide, unused alkali, unchanged chromite and sodium chloride. Extraction of the roast with hot water yields a pH of 10-11, and the pH is controlled so that the chromate dissolves and the alkali-soluble impurities hydrolyze and form a filterable precipitate. This process is inefficient at leaching all of the chromate from the COPR during processing. Consequently, total Cr(VI) concentrations in the soils/solids at COPR impacted sites can range into the single-digit percent by weight range (with one percent by weight equaling 10,000 mg/kg). This includes a fraction that is readily susceptible to leaching and a fraction that is incorporated into agglomerated solids and/or the various minerals comprising the COPR solids. Thus, the residual Cr(VI) associated with COPR is both a long term threat to the environment due to mobility, and hard to treat due to the challenge of access within the COPR solids. The elevated concentrations can also inhibit effective bioremediation due to toxicity.
Highly Alkaline.
Materials with significant concentrations of COPR are characteristically alkaline, typically exhibiting a pH greater than 12. This is buffered over the long term by the slow hydration of the primary oxide minerals in the COPR into hydroxides, specifically dissolution and secondary precipitation of calcium aluminate phases. Solid phases in COPR that contain Cr(VI) are chromium(VI) hydrocalumite (Ca4Al2(OH)12CrO4.6H2O) and Cr(VI)-substituted hydrogarnet (Ca3Al2(H4O4)3). When the pH decreases below 11.2, chromium(VI) ettringite (Ca6Al2(OH)12—(CrO4)3.26H2O) is likely to be present.
Minerals, in addition to chromite, that have been identified in COPR include periclase (MgO), brucite (Mg(OH)2), and calcite (CaCO3) derived from the addition of lime and dolomite; and brownmillerite (Ca2(Al,Fe,Cr)2O5), which is associated with Cr(III) (See Geelhoed, J. S., Meeussen, J. C. L., Roe, M. J., Hillier, S., Thomas, R. P., Farmer, J. G., and Paterson, E., 2003, Chromium Remediation Or Release? Effect Of Iron(II) Sulfate Addition On Chromium(VI) Leaching From Columns Of Chromite Ore Processing Residue, Environmental Science and Technology 37: 3206-3213). Buffering of the pH by COPR is dominated by the dissolution of Ca-containing phases. The alkaline pH created by many of these minerals can impede treatment by chemical reductants such as ferrous iron, by encouraging oxidation of the iron thereby limiting its distribution. Because oxygen can be an effective oxidant of Cr(III) at high pH, if the alkalinity is not overcome the potential for long-term oxidation of precipitated Cr(III) back to Cr(VI) can increase based on residence time, availability of moisture, and abundance of oxygen.
Susceptibility to Heaving.
As the COPR minerals weather/hydrate, some of the hydration products can result in lithification/cementation at various scales. As the weathering continues, mineral grains trapped in a cemented matrix will decrease in density and increase in volume (See Dermatas, D., Chrysochoou, M., Moon, D. H., Grubb, D. G., Wazne, M., and Christodoulatos, C., 2006, Ettringite-Induced Heave In Chromite Ore Processing Residue (COPR) Upon Ferrous Sulfate Treatment. Environmental Science and Technology 40: 5786-5792). This is important in the case of ettringite [Ca6(Al(OH)6)2.(SO4)2.26H2O], a known expansive mineral both in the cement and soil literature, and delayed ettringite formation (DEF) is one of the main mechanisms of concrete deterioration (See Taylor, H. F. W., Famy, C., 2001, Scrivener, K. L. Review Delayed Ettringite Formation, Cement and Concrete Research, 31, 683-693). Depending on the magnitude of the bulking compared to the available porosity and the degree of rigidity exhibited by the surrounding matrix, this can result in surface heaving. This is not an issue at all COPR sites, but is an important consideration relative to the re-development potential of affected properties.
A majority of the existing treatment approaches for COPR involve the use of chemical reductants to reduce the Cr(VI) to Cr(III). Such approaches require significant amounts of costly reagents to account for the mass of Cr(VI). A common reductant for Cr(VI) is ferrous iron, supplied in the form of ferrous sulfate to promote the following reaction (Eary, L. E., and Rai, D., 1988, Chromate Removal From Aqueous Wastes By Reduction With Ferrous Iron, Environmental Science and Technology 22: 972-977):Cr(VI)(aq)+3Fe(II)(aq)=Cr(III)(aq)+3Fe(III)(aq)
This reaction ignores all of the possible complexes of aqueous Cr and Fe, however Cr(III) rapidly hydrolyzes to form insoluble Cr(OH)3 at circumneutral pH, as does Fe(III) to precipitate as iron oxyhydroxide. An important consideration in the use of ferrous sulfate is the poor solubility of this reagent at circumneutral pH and under oxic conditions; and extremely poor solubility at alkaline pH and highly oxidizing conditions, characteristic of the COPR. Therefore, delivery and distribution can be challenging when using this reagent.
Work in the lab by Geelhoed et al. with COPR from Glasgow, UK (10,000 mg/kg Cr(VI)) showed that the addition of 1 g/L ferrous iron as ferrous sulfate was ineffective for treatment due to the immediate precipitation of iron in response to the highly alkaline conditions. In addition, sulfate displaced chromate from hydrocalumite (Ca4Al2(OH)12CrO4.6H2O).
Similarly, field application of solid ferrous sulfate (30-50 wt. %), mixed into COPR (4,000 mg/kg Cr(VI)) with augers resulted in incomplete treatment of the Cr(VI) and did not result in neutralization of the alkaline minerals in the COPR (See Dermatas). In addition, this method was determined to be uneconomical because of the high concentration of ferrous sulfate that was required.
Work by Su and Ludwig summarized a field test involving injection of 5700 L of a 0.07 M FeSO4+0.07 M Na2S2O4 solution into a COPR saturated zone (pH 11.5) indicated no well and formation clogging during injection. (See Su, C., and Ludwig, R. D., 2005, Treatment Of Hexavalent Chromium In Chromite Ore Processing Solid Waste Using A Mixed Reductant Solution Of Ferrous Sulfate And Sodium Dithionite, Environmental Science and Technology 39(16): 6208-6216). Examination of a core collected 0.46 m from the injection well following injection indicated effective treatment of the solid phase Cr(VI) based on analysis of water, phosphate solution, and high temperature alkaline extracts. The combined reductant solution also imparted a residual treatment capacity to the COPR allowing for subsequent treatment of dissolved phase Cr(VI); however, dissemination of the iron in the highly alkaline environment appeared to be impeded by the inability to sufficiently lower the pH with distance from the injection well to avoid precipitation of Fe(OH)2 and likely also FeCO3. Injection of a 0.2 M FeSO4+0.2 M Na2S2O4 solution into another COPR saturated zone (pH 9) indicated much more effective dissemination of the injected iron. Post-treatment Scanning electron microscopy—energy dispersive x-ray spectroscopy analyses of post treatment core samples indicated that much of the Cr(VI) may be removed through the formation of a Cr-bearing precipitate, possibly a complex carbonate, characterized by a Fe:Cr molar ratio of roughly 1:1 (See Ludwig, R. D., Su, C., Lee, T. R., Wilking, R. T., and Sass, B. M., 2008, In Situ Source Treatment Of Cr(VI) Using A Fe(II)-Based Reductant Blend: Long-Term Monitoring And Evaluation, Journal of Environmental Engineering 134(8): 651-658).
An even more stable solubility controlling phase for Cr is the Fe—Cr hydroxide solid solution (Sass, B. M., and Rai, D., 1987, Solubility Of Amorphous Chromium(III)-Iron(III) Hydroxide Solutions. Inorganic Chemistry 26: 2228-2232):CrO42−+3Fe2++8H2O→4Fe0.78Cr0.25(OH)3(s)+4H+
The solubility of the mixed Fe—Cr hydroxide is two orders of magnitude lower than the Cr hydroxide solid and the creation of this phase is desirable for achieving COPR treatment with long-term stability relative to decreased leaching of Cr.
Other means of reducing Cr(VI) to Cr(III) involve the use of elemental iron, iron sulfide, organic compounds, microbial activity, and sulfide, as detailed here.
U.S. Patent Application Publication No. US2004/0126189 titled “Method for stabilizing chromium-contaminated materials” prepared by A. K. Chowdhury discloses an in-situ method that uses the sulfuric acid/ferrous iron solution created from the oxidization of pyrite for treatment of COPR. The method involves application of the iron and acid to the top of a column of COPR.
U.S. Pat. No. 5,304,710 titled “Method of detoxification and stabilization of soils contaminated with chromium ore wastes” and issued to Kigel, M. Y., et al., discloses and ex-situ method that involves the acidification of the COPR to pH 3 using sulfuric acid, followed by treatment with a ferrous sulfate solution, and then once the Cr is reduced, raising the pH back up to pH 7.5 to 8.2 with cement or cement kiln dust. The acidification step for this process is performed in order to speed up the kinetics of Cr(VI) reduction by ferrous iron, and to maintain the ferrous iron in the dissolved form.
U.S. Pat. No. 7,452,163 titled “Method of treatment, stabilization, and heave control for chromite ore processing residues (COPR) and chromium contaminated soils” and issued to Wazne, M., et al., discloses a process that considers both the minerals that dissolve rapidly in response to acid addition, and those that dissolve more slowly over time resulting in pH rebound to alkaline conditions. Strong mineral acid (hydrochloric acid) is applied to the COPR along with water as a heat sink for the exothermic reaction. Ferrous iron is also described as beneficial for pH neutralization because of the protons (H+) that are released when ferrous iron hydrolyzes or oxidizes to ferric iron. The heave potential is ameliorated by decreasing the pH to below 10, where the ettringite matrix is transformed to calcite, gypsum, and amorphous alumina. Sulfate is used to complete the transition from brownmillerite to ettringite, to consume aluminum and incorporate it into ettringite. Chemical reductants are added (ferrous chloride, ferrous sulfate, calcium polysulfide, or sodium bisulfide) when the pH is less than 10. Finally, the treated COPR is mixed with asphalt to encapsulate it, or with barium hydroxide to further control chromium leaching.
U.S. Pat. No. 5,562,588 titled “Process for the in situ bioremediation of Cr(VI)-bearing solids” and issued to Thomas E. Higgins discloses a process that incorporates a solid or semi-solid organic material containing bacteria, nutrients, and mineral acid to maintain the pH between 6.5 and 9.5 to promote biochemical reduction of Cr(VI) to Cr(III) in-situ. This process does not account for the potential toxicity of very high concentrations of Cr(VI) in the COPR that can inhibit microbial growth and effective bioreduction of Cr(VI). In addition, the claim details instances of poor microbial growth, likely due to the combination of the addition of organic material and mineral acid.
Experience with other reductants includes polysulfide application in the field. Moon et. al. applied acid and 10-20 wt. % solutions of calcium polysulfide. (See Moon, D. H., Wazne, M., Dermatas, D., Christodoulatos, C., Sanchez, A. M., Grubb, D. G., Chrysochoou, M., and Kim, M. G., 2007, Long-Term Treatment Issues With Chromite Ore Processing Residue (COPR): Cr6+ Reduction And Heave, Journal of Hazardous Materials 143(3): 629-635). The results were incomplete treatment (40% of Cr(VI) in the COPR was reduced) and little change in the mineralogy of the COPR. Once the temporary reducing capacity was overwhelmed there was a rebound in Cr(VI) concentrations.